1. Field of the Invention
This invention relates to a disc-like recording medium represented by a magneto-optical disc and a magnetic disc, and particularly to a disc-like recording medium in which pre-formatted data such as clock information, servo information and read-only information are formed in advance and an apparatus for producing the disc-like recording medium by which the pre-formatted data are formed in the recording medium in advance.
2. Description of the Related Art
A technique of generating a system clock, from a clock mark buried in a disc, which is necessary for operating a disc driving apparatus has been practically employed for sample servo format in the magneto-optical disc.
An arrangement of a magneto-optical disc of the conventional sample servo format and an arrangement of a magnetic disc of the conventional sample servo are shown in FIGS. 1 and 2, respectively. Both of these discs are provided with a data area 101 in which information signals are recorded and a servo area 102 in which servo information is recorded in advance. In the servo area 102, so-called pre-formatted data are formed such as servo marks 103, clock marks 104, and access codes and read-only information (ROM information) 105.
For recording the pre-formatted data in the magneto-optical disc, a disc substrate 110 is provided with pits corresponding to the pre-formatted data when it is molded, and a reflecting film or a vertical magnetization film (recording film) 111 is formed thereon, as shown in FIG. 3. Thus, viewed from the side of the disc substrate 110, substantially circular lands are formed, that is, the servo marks and the clock marks.
As for recording of the pre-formatted data in the magnetic disc, there are two methods. By one method, the disc substrate is provided with pits corresponding to the pre-formatted data when it is molded, similarly to the above and as shown in FIG. 4A. After a magnetic film 112 is formed on the disc substrate 110, pits and lands in the servo area are magnetized in different directions. Thus, the magnetized information is formed as the servo marks and the clock marks.
By the other method, the magnetic film 112 is formed on the entire flat surface of the disc substrate 110, as shown in FIG. 4B, After a portion of the magnetic film 112 corresponding to the pre-formatted data is partly removed using a known lithography technique, the magnetic film in the servo area is magnetized. Thus, the residual portions of the magnetic film or the removed portions are formed as the servo marks and the clock marks.
Particularly in the above magnetic disc, the clock marks are formed continuously in the radial manner and direction of the disc so that the clock marks may be satisfactorily reproduced even though the head gap of the magnetic head has off-tracked.
System clock signals (recording/reproducing clock) may be produced by multiplying a clock pulse which is produced for each passage of the optical pickup on the clock mark 104 on the magneto-optical disc or of the magnetic head on the clock mark 104 of the magnetic disc.
Conventionally, the clock marks 104 are formed to be arrayed radially and linearly from the center of the disc. By this sample servo format, the recording/reproducing clock which is synchronized with the phase of the clock pulse produced from the clock mark 104 is generated using a phase locked loop (PLL). The PLL includes a phase comparator 121, a low-pass filter (LPF) 122, a voltage control oscillator (VCO) 123 and a frequency divider 124 for dividing the frequency by n, as shown in FIG. 5. The clock pulse Sc is entered to the phase comparator 121, thus initiating the oscillation of the VCO 123 at a frequency which is n times higher than that of the clock pulse Sc. Therefore, the recording/reproducing clock Ss having the frequency which is n times higher than that of the clock pulse Sc can be produced.
This recording/reproducing clock Ss is operative to grasp a .theta.-coordinate on the disc and to carry out recording and reproduction of all data. Accordingly, normal recording and reproduction may be carried out as long as the recording/reproducing clock Ss synchronized with the clock pulse Sc is produced. However, once the synchronization of the recording/reproducing clock Ss with the clock pulse Sc is released, normal recording and reproduction cannot be carried out at a predetermined timing.
Thus, conventionally, a linear head actuator (positioner) is used which linearly shifts the recording/reproducing head in the radial direction of the disc in order to access a desired track. By using this linear head actuator, the radial array of the clock marks 104 is in conformity with the direction of shift of the head actuator. Therefore, even in accessing the desired track at a high rate, the clock pulse Sc can be produced at equal time intervals, thus avoiding the disturbance to the PLL. That is, the phase of the recording/reproducing clock Ss will not be released from the phase of the clock pulse Sc.
The above mechanism is described with reference to FIGS. 6A and 6B. FIG. 6A shows the moment at which the clock mark indicated by point A is reproduced in the inner track, while FIG. 6B shows the reproduction of the next clock mark indicated by point B at the time when the outer track is accessed.
As seen from FIGS. 6A and 6B, the time since the reproducing head reproduces an arbitrary clock mark until it reproduces the next clock mark is equal to the time for the disc to rotate only an angle .theta.1 which is formed by two adjacent clock marks. Consequently, as long as the disc is rotated at a constant speed, no phase difference is generated between the recording/reproducing clock Ss and the clock pulse Sc even at the time of the access. Therefore, it is possible to reproduce or record data during the access.
However, the linear head actuator as conventionally used has disadvantages such that when the data in the disc are intended to be accessed at, a high speed, a large quantity of current is needed while the tracking is likely to be disturbed by externally applied acceleration.
It is appreciated from the above that the use of rotating type head actuator which is provided with the head on the distal end of a rotating arm thereof is advantageous for being unaffected by the increase in the access speed and the externally applied acceleration.
However, as the locus m of the head of the rotating head actuator draws an arc, as shown in FIG. 7, the head has a circumferential velocity component Vt at the time of the access. Therefore, a problem is generated that the locus m of the head is not along the radial direction of the disc when accessing data in the conventional disc, that is, the disc in which the pre-formatted data such as clock marks are formed radially and linearly.
For this reason, even though the disc is rotated at a constant velocity, the time interval of the clock pulse Sc produced from the disc changes in accordance with the access speed of the head. The PLL for generating the recording/reproducing clock Ss is operative to reduce the phase difference between the clock pulse Sc and the recording/reproducing clock Sc. However, a phase deviation transiently remains with a finite gain and band.
FIG. 8 shows temporal changes of the phase difference between the clock pulse Sc and the recording/reproducing clock Ss at the time when the rotating head actuator has accessed data on the disc having the clock marks 104 arrayed radially and linearly therein. In this example, a negative phase deviation is generated when the actuator is accelerated, while a positive phase deviation is generated when the actuator is decelerated. These phase deviations will not be zero for a while even after the end of the access. Such phase differences between the clock pulse Sc produced from the disc and the recording clock Sc prevent the recording and reproduction of data. Thus, it is impossible to read the track address during the high-speed access and to start the recording/reproduction of data at the same time as the end of access.
That is, as seen from FIGS. 9A and 9B, on the assumption that the head which is mounted to the rotating head actuator meets one clock mark at point C and then meets the next clock mark at point D during its outward access, the angle of rotation .theta.2 of the disc in this access is smaller than the rotation angle .theta.1 shown FIG. 6. Accordingly, the time taken from the reproduction of the clock mark until the reproduction of the next clock mark is reduced.
On the contrary, in case the data is accessed from the outer circle toward the inner circle of the disc, not shown, the time since the reproducing head reproduces an arbitrary clock mark until it reproduces the next clock mark is increased. In addition, the time interval changes in accordance with the access speed. The changes in the time interval for reproducing the clock marks cause the phase deviation of the PLL.
This problem remains also in case the magnetic disc is used in which the pre-formatted data, such as clock marks 104, servo marks 103 and ROM information 105, are radially and linearly formed in advance, as shown in FIG. 10. Particularly when this magnetic disc is used, the following problem as well as the above problem is generated. That is, when the magnetic disc is used in which the pre-formatted data, such as the clock marks 104, the servo marks 103 and the ROM information 105, are radially and linearly formed, as shown in FIG. 10, an azimuth angle .beta. is generated between the edge of the pre-format and the head gap g by an angle of rotation of the rotating head actuator, thus excessively lowering the reproducing output.